The following present disclosure is provided in relation to Proton Exchange Membrane (PEM) stacks. The force distributor according to the invention can however, also be used for other types of fuel cells such as SOFC fuel cell stacks. Molten Carbonate Fuel Cells (MCFC) or Direct Methanol Fuel Cells (DMFC). Further, the invention can also be used for electrolysis cells such as Solid Oxide Electrolysis Cells and such cell stacks. The electro-chemical reactions and the function of a fuel cell or an electrolysis cell is not the essence of the present invention, thus this will not be explained in detail, but considered known for a person skilled in the art.
In a traditional fuel cell stack (as shown in FIG. 1), a plurality of fuel cell units 112 and traditional flat end plates 110 are assembled to form a stack 114. It is understood that a UEA 116 may be disposed between a pair of fuel cell plates (bipolar plates) thereby forming a fuel cell unit among the other similarly constructed fuel cell units schematically represented by phantom lines 115. The UEA 116 may include diffusion mediums (also known as a gas diffusion layer) disposed adjacent to an anode face and a cathode face of a membrane electrolyte assembly (MEA). The MEA includes a thin proton-conductive, polymeric, membrane-electrolyte having an anode electrode film formed on one face thereof, and a cathode electrode film formed on the opposite face thereof. In general, such membrane-electrolytes are made from ion-exchange resins, and typically comprise a perfluoronated sulfonic acid polymer such as NAFION™ available from the E.I. DuPont de Nemeours & Co. The anode and cathode films, on the other hand, typically comprise (1) finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material (e.g., NAFION™) intermingled with the catalytic and carbon particles, or (2) catalytic particles, sans carbon, dispersed throughout a polytetrafluoroethylene (PTFE) binder.
The efficiency of the fuel cell stack 114 is dependent of small contact resistance between the various UEA's 116 and bipolar plates 118, and therefore it is crucial that a suitable compression force is applied to the fuel cell stack. This compression force must be large enough and evenly distributed throughout the area of the fuel cell stack 114 to ensure electrical contact, but not so large that it damages the electrolyte, the electrodes, the electrical interconnect or impedes the gas flow over the fuel cell. The compression of the fuel cell stack 114 is also vital for the seal between the layers of the stack to keep the stack gas tight. Further, different areas of the cell stack may require different compression forces, the electrochemical active (GDL—Gas Diffusion Layer) area of the cell stack 114 may require a higher compression force than the sealing areas (peripheral region of the bipolar plates). More importantly, the stack-up tolerance of the UEA's, bipolar plates and seals may change force distribution between the active area (GDL) area and seal area leading to compression force on the seal being either too high or too low. Hence, not only must the compression force be evenly distributed over some areas, there can also be a need to tailor the compression force such that a compression force of a first magnitude is evenly applied to some surface areas of the cell stack, but a compression force of a second, third and more magnitudes is evenly applied to other surface areas of the cell stack according to the specific compression requirements of the area in question.
A solution to this problem has been proposed in WO 2008089977 describing how the fuel cell stack has thermally insulating end blocks having one rectangular planar side facing the stack and an opposing side of convex shape. Springs tighten a flexible sheet against the convex shaped face of the end blocks, whereby the spring force is evenly distributed over the stack end areas.
In DE 10250345 a housing surrounding a SOFC is provided and a compressible mat between the stack and the housing provides a compression force to the cells both radially and axially.
WO 2005045982 describes how a multi-function end plate assembly may be used to preferentially compress a region of the fuel cell stack.
In WO 2008003286 a stack is compressed by thermally insulating elements, which are pressed against the stack by an elastic sleeve. The sleeve can for instance be made of silicone.
However, the aforementioned references are rather expensive to produce given the complexity of each design and the number of parts. Moreover, the aforementioned references disclose designs wherein uneven loads can still occur during operation and start/stop periods due to the changing conditions of especially temperature and pressure. Unevenness can lead to damage or performance reduction of the fuel cell stack.
Also, as is known, the traditional flat end plate 110 (shown in FIG. 1) cannot adjust forces of different magnitude to different areas of the cell stack end surface. Hence, the compression force cannot be tailored to the different requirements of different areas of the cell stack when the stack-up tolerance is involved.
Accordingly, there is a need to provide a robust fuel cell end plate design which redistributes the loads across the fuel cell stack in specific areas while at a reduced cost.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. Accordingly, there is a need for an improved end plate unit for a fuel cell stack which better distributes compression loads across the fuel cell stack.